High fidelity audio amplifier



- March 4, 1958 s. A. C QRDERMAN 2,825,766

HIGH FIDELITY AUDIO AMPLIFIER Filed June 30, 1955 IN VENTOR Unite ite The present invention relates generally to amplifiers capable of prodding wide band audio amplification, at high output power and high efficiency.

i is a broad object of the present invention to provide a novel high fidelity audio amplifier.

it is another object of the present invention to provide a novel amplifier having negative feedback deriving from an output transformer secondary Winding which is bifilarly related to a main secondary winding, so that feed-back signal accurately represents or duplicates that applied to a load.

Still another object of the present invention relates to the provision of a power output stage of the push-pull high elilciency type, in which the cathode of each tube of a push-pull pair is maintained at the same A. C. potential as the anode of the other tube of the pair, by means of two bifilarly related windings, one of which is disposed between the cathodes of the tubes and the other between the anodes, and in which the power output stage is driven from a push-pull cathode follower stage which is driven at its various electrodes from the bi-filarly related windings in such phases and amplitudes as to compensate for the cathode swings of the pushpull pair.

It is still another object of the present invention to provide a push-pull amplifier output stage having cathode and anode loading, and in which each cathode of a push-pull output tube pair is maintained at the same A. C. potential as the anode of the remaining tube, and in which the output stage is driven from push-pull cathode followers the anodes of which are maintained at the A same A. C. potentials as the cathodes of the driven tubes,

the push-pull cathode followers being in turn driven from a push-pull anode-loaded voltage amplifier stage having anode voltages compensated for anode drive of the cathode follower stage.

The above and still further features, objects, and advantages of the invention will become apparent upon consideration of the following detailed description of a specific embodiment of the invention, especially when taken in conjunction with the accompanying drawings,

wherein:

The single figure of the drawings as a schematic circuit diagram of an amplifier in accordance with the invention.

Referring now more particularly to the accompanying drawings, the reference numeral 1 denotes an input terminal for the amplifier of the present invention. The terminal 1 applies input voltage across a potentiometer 2, having a variable tap 3 for gain control purposes.

The variable tap 3 is connected to the grid of a single ended amplifier stage 4, including a triode 5 having a condenser bypassed bias resistor 6 in its cathode circuit, in series with an tin-bypassed resistor 7, the latter being utilized for applying negative feed-back to the triode 5.

The anode of triode 5 is directly connected to one grid 8 of a twin triode 9. The anode of the triode 5 is supplied with voltage from a conventi .nnl rectifier atet Patented Mar. 4. 1958 ltQQ supply ltd, via lead 11 and one anode load resistance 12, and a further anode load resistance 13, in series.

The other grid 14 of the twin triode is connected at the junction of resistors 12 and 13, so selected that grid 14- is at about the same D. C. potential as is the grid 8. The grid 1a is filtered for audio signals by an RC filter 15, and the triode section containing the grid 14 is driven its cathode, by virtue of audio signal developed across cathode resistor 15. The anodes of twin triode 9 are supplied with anode voltage via lead ll, and anode load resistors l7, 13.

The twin-triode 9 operates as a phase splitter, in accordance with well-hnown principles, and of itself forms no part of the present invention.

The voltages at the anodes of twin triode 9, of respectively opposite phase, are applied to the control electrodes of a pair of triodes 2%, 21, by means of the usual RC coupling network, the cathodes of triodes 26, 21 having a common resistance 22, to ground.

The twin triodes Zll, 21 are anode loaded, by resistors 23, 24, respectively, and the anodes are coupled via condensers 25, 2d, and grid leaks 27, 28, to the control grids of triodes 29, 3d. The latter operates as cathode follower stages, having cathode resistors 31, 32, respectively.

The voltages developed across cathode resistances 31, 32 are applied to the control grids of a push-pull output stage, comprising tetrodes 3d, as.

An output primary winding 37 is connected between the cathodes of the tetrodes 35;, 36, and the center point of primary winding 37 is grounded. Associated with the primary winding 37 is a further primary winding 38, bifilarly related to the winding 37, so that the adjacent ends of the windings 37, 38 are always at the same A. C. potential, for frequencies as low as fractional parts of a cycle per second and as high as several hundred thousand cycles per second.

identifying the respective cathode terminals of winding 37 by the reference numerals 49, 4-1, and the respectively adjacent terminals of winding 38 by reference numerals 42, 13, the terminal 42 is connected to the screen grid of the tetrode 35 and the anode of tetrode 36, while terminal 43 is connected to the screen grid of the tetrode 36 and to the anode of tetrode 35. A center tap of winding 30 is connected to power supply 10, via lead 44, and supplies anode and screen voltage to tetrodes 35', and 35.

In operation, then, the anode of tetrode 36 remains at the same A. C. potential as the cathode of tetrode 35, and the anode of tetrode 35 remains at the same A. C. potential as the cathode of tetrode 36. The screen of each tetrode remains at the same A. C. potential as its cathode, and the same D. C. potential as its anode. It is known that the described connection of the output stage results in high efiiciency and substantially distortionless amplification over an extremely wide band of frequencies, from well below to well above the audio band.

An output winding 56 is provided, having two subwindings, 51 and 52, wound bifilarly with respect to each other. The sub-winding S1 is employed as a low irnpedance signal output winding, i. e. to drive a loud speaker, or the like, and by virtue of its winding relation to windings 3'7, 3% is capable of responding precisely to the signals present in these windings, and over the same frequency band.

The sub-winding 52 is connected across cathode resistance 7 of triode 4, and supplies a negative feed-back which precisely follows the current in winding 51, by reason of the recited bifilar winding relation.

it will now be noted that the cathodes of the tetrodes 35, 36 are strongly driven. In order to provide an excess driving voltage at the control grids of tetrodes 35 36, the anodes of triodes 29, are driven directly from terminals 42, 4-3, thereby supplying not only a D. C. voltage to the anodes, but also an A. C. component of voltage precisely equal to that at the cathodes of tetrcdes 35, 36. The net drivingvoltageat the control grids of tetrodes '35, 36 is thus precisely that which would exist across cathode loads 31, 32, were they disconnected from tetrodes 35, 36.

In order to compensate for the D. C. voltage across resistors 31, 32. due to D. C. anode current, the junction of resistors 31, 32 is maintained at a negative voltage, derived via lead 55 from diode dry rectifier 56, energized from one half the power input transformer secondary winding 57. The net D. C. voltage and the control electrodes of tetrodes 35, 36 may be thus selected to have grid bias values suitable for class B or A3 operation, as desired, while triodes 29, 3% are driven class A.

A resistance 58 may be connected between the junctions of resistors 31, 32, and of resistors 27, 28 and a fourth resistance 59, from the latter junction to ground. The resistances 58, 59 establish the bias with respect to ground of the control electrodes of triodes 29, 3-8, which are designed to be operated as class A push-pull voltage amplifiers.

The use of cathode follower drivers for the partially cathode loaded output stage of the amplifier introduces a further problem in that the drivers of the cathode follower triodes 29, 30 are required to overcome the A. C.

bias present at the cathodes of triodes Zfi, as well as to supply excess driving voltage to the control grids of triodes 29 and 39. The anode load resistances 23,

.24 of triodes 2&21 are accordingly supplied not only with D. C. voltage, but also with audio voltage, and not alone with D. C. voltage, as is conventional. The anode resistors 23, 24 are, to this end, supplied with voltage identical with that supplied at the anodes of triodes 29, 3%. The A. C. component of this voltage is supplied to the grids of triodes 29, via coupling condensers 25, 26, and in suitable amplitude to counter balance the A. C. voltage at the cathodes of'triodes 29 and 30. The net effect, then, is that the anodes, cathodes and control grids of triodes 29, 3t and the anodes of triodes 29, 21 are A. C. driven with replicas of the voltage at the cathodes of tetrodes 35, 36. Thereby the tetrode drive circuits are completely compensated for the cathode volt age variations of tetrodes 35, 36.

While I have described and illustrated one specific example of the present invention it will be clear that variations of the specific details of construction may be resorted to without departing from the true spirit of the invention as defined in the appended claims.

What I claim is:

1. An amplifier including an output stage, and a cathode follower driver stage, said output stage including a first vacuum tube having a first anode, cathode, and control grid and a second vacuum tube having a second anode and cathode and control grid, means connecting said first and second tubes in push-pull relation, a cathode load for each of said first and second tubes, an

anode load for each or" said first and second tubes, means including said cathode loads and said anode loads for maintaining the cathode of said first tube at the same alternating potential as the anode of the second tube and the cathode of the second tube at the same alternating ode, and means for driving said last named anodes, control electrodes and cathodes in response to voltage developed across said anode loads.

2. The combination in accordance with claim 1 wherein is provided a D. C. connection between said first anode and said third anode, and a further D. C. connec tion between said second anode and said fourth anode, a D. C. connection between said third cathode and first control grid, and a further D. C. connection between said fourth cathode and said second control grid.

3. The combination in accordance with claim 2 wherein is further provided a voltage amplifier balanced driver stage for said third and fourth vacuum tubes, said voltage amplifier including a pair of resistive anode loads, and means for energizing said resistive anode loads and said voltage amplifier in series with said first mentioned anode loads.

in an amplifier, an output stage having a first amplifier tube, said first amplifier tube having a first anode, cathode and control electrode, means for loading the cathode circuit of said amplifier tube, whereby the potential of the cathode of said amplifier tube varies at A.'C. potential while said amplifier tube delivers A. C. output signal, a cathode loaded driver stage for said first amplifier tube, said driver stage including a second amplifier tube, said second amplifier tube having a second control grid, anode and cathode, a resistive cathode load circuit for said second amplifier tube, a direct connection between said second cathode and said first control electrode, means responsive to'the output signal of said output stage for driving said second anode in such magnitude and phase as to tend to equalize the potentials of said second cathode and said first cathode and means tending to equalize the potentials of said second control grid and said first cathode.

5. In combination, a push-pull output amplifier stage, said amplifier stage including a first and second amplifier tube having respectively first and second cathodes, anodes, and control grids, means for biasing said control grids for alternate operation and cut-oil of said amplifier tubes in response to an A. C. signal, a balanced cathode primary output winding connected between said first and second cathodes, an anode winding unity coupled to said cathode Winding, said anode winding having one terminal unity coupled to said first cathode and a second terminal unity coupled to said second cathode, a connection between said one terminal and said second anode, a connection between said second terminal and said first cathode, a center anodevoltage terminal for said anode winding, a push-pull cathode loaded driver amplifier stage for said push-pull output amplifier stage, said push-pull cathode loaded driver stage including a third and a fourth amplifier tube, said third and fourth amplifier tubes having respectively third and fourth anodes, cathodes, and control electrodes, means connecting said first control grid to said third cathode, means connecting said second con trol grid to said fourth cathode, means connecting said first terminal to said third anode, means connecting said second terminal to said fourth anode, a plate loaded driver stage for said cathode follower, said plate loaded driver stage including a fifth and a sixth vacuum tube, said fifth and sixth vacuum tubes including respectively fifth and sixth anodes, cathodes and control electrodes, a separate resistive load for each of said fifth and sixth anodes, a separate coupling network between each of said fifth and sixth anodes, respectively, and said third and fourth control grids respectively, a direct connection in series between said first terminal and said fifthanode via the resistive load of said fifth anode, a directconnection from said second terminal and said sixth anode via the resistive load of said sixth anode, and means for driving said fifth and sixth control electrodes in balanced relation.

6. The combination in accordance with claim 5 wherein is provided a single D. C. source of voltage for D. C.

a biasing both said first and second cathodes and said third and fourth control electrodes and cathodes for said alternate operation of said first and second amplifier tubes. 7. In an amplifier, a first vacuum tube amplifier having a first cathode winding grounded at one end, a further winding bifilarly related to said cathode winding, whereby adjacent and terminals of the first and further windings are maintained at identical A. C. potential over a Wide range of frequencies, a source of D. C. plate voltage connected in series With said further winding at its point of A. C. ground potential, a driver amplifier tube for said first vacuum tube amplifier, said driver amplifier tube being cathode loaded, means directly connecting the cathode of said driver amplifier tube to the grid of said first vacuum tube and means for connecting the anode of said driver tube directly with a point of said further Winding which maintains the same A. C. potential as the cathode of said first vacuum tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,161,844- Babler dune 13, 1939 2,379,168 McClellan June 26, 1945 2,386,892 Hadfield Oct. 16, 1945 2,516,181 Bruene July 25, 1950 2,629,395 Oliver Feb. 17, 1953 2,646,467 McIntosh July 21, 1953 2,654,058 McIntosh Sept. 29, 1953 

